In recent years, in order to deal with the global warming problem, a power generation system by a solar cell has become rapidly widespread. Also, efficiency of the power generation system has been enhanced, and operation equipment thereof has been improved to be stabilized. Therefore, in the case where a scale of power generation equipment is large, measurement equipment to be associated therewith becomes necessary, and performance to save power and allow a stable operation is required also for supply of a power supply to the measurement equipment.
Heretofore, in the case where a power supply voltage on a supply side that operates the measurement equipment and a voltage of the power supply necessary for the measurement equipment and the like are different from each other, a value of the power supply voltage has been stepped down by using a serial-type constant voltage circuit or a constant voltage circuit including a switching element having good power conversion efficiency.
For example, in Patent Document 1, a direct-current stabilized power supply device is disclosed, in which a serial-type constant voltage circuit and a stabilizing circuit including a switching element are combined with each other. FIG. 8 is a schematic diagram of the direct-current stabilized power supply device described in Patent Document 1.
In FIG. 8, a rectifier circuit 10 rectifies AC 100V, and supplies a switching power supply circuit 11 with a current as an input voltage thereto. The switching power supply circuit 11 compares, by a comparator 133, a triangular wave, which is generated by a triangular wave generation circuit 132, with an output voltage of an inductor 135 and an output voltage of a three-terminal regulator 12. In such a way, the switching power supply circuit 11 creates a necessary switching control signal, and drives a switching element 131. Specifically, the voltage that is dropped once in the switching power supply circuit 11 is further converted by the three-terminal regulator 12 into a low voltage, which is necessary for a power supply load RL, and a current is then supplied thereto.
However, in such a conventional configuration viewed in Patent Document 1, in the case where a voltage of photovoltaic power generation exceeds 500V, there is no margin in high withstand voltage performance required for the switching element, and further, large power is necessary for a control circuit for the switching element at the time of such a high voltage, leading to complexity of the circuit and requirement for performance of circuit elements, and accordingly, it is difficult to realize this configuration.
Moreover, in Patent Document 2, it is disclosed that, in an unstable state such as cloudy weather, a power supply device using a solar cell output is combined with a battery. However, in this configuration, the battery in which a lifetime is restricted in terms of the number of charge/discharge times cannot be used for a measurement device for use in a photovoltaic power generation device for which a long lifetime is expected.
Furthermore, in Patent Document 3, it is disclosed that, in the case where the power supplied from the solar cell to the power supply falls short, a power is supplied from a commercial power supply in parallel thereto. However, in this configuration, the commercial power supply is used, and accordingly, in particular in large-scale photovoltaic power generation equipment, it costs enormously to wire a power supply line, and extra power is consumed.
Moreover, in Patent Document 4, a power supply device is disclosed, which supplies power of a high voltage through a switching element to a low-voltage load. However, in this configuration, a similar problem to that in Patent Document 1 is still present.